A wire harness mounting structure includes: a clamp including a body part, an engaging part, and a supporting part; and a mounting part having a through hole. The supporting part is smaller than the engaging part as viewed in a height direction along a facing direction of the body part and the engaging part. The through hole includes: a first through hole larger than the engaging part as viewed in the height direction; a second through hole that is continuous with the first through hole and is smaller than the engaging part and is larger than the supporting part as viewed in the height direction; and an elastic piece that is arranged in the first through hole. The second through hole has a shape in which only the supporting part enters therein when the engaging part enters the first through hole while deforming the elastic piece.